The present invention concerns a high-temperature lubricant for the hot processing of metals.
In the hot working of metals, in particular steel, in a temperature range of 700-1300° C., scale formation occurs at the heated metal surface, at the ambient air. In the case of hot rolling methods for steel for the production of seamless tubes and pipes, a solid material is pierced and a hollow block formed, which is then elongated in subsequent rolling steps. Here the danger of scale formation on the heated metal surface of the hollow block during transfer to the elongation procedure is particularly high. In the subsequent rolling steps that occurrence of scale can lead to internal flaws in the seamless tube or pipe. For that reason the scale involved is blown off for example with compressed air or inert gas. In addition very widely differing substances in powder form are applied as lubricants or mordant agents to the inside surface of the hollow blocks. Examples of such lubricants or mordant agents contain graphite, boron nitride, molybdenum sulphide, silicates or alkaline earth phosphates and mixtures thereof.
Many lubricants for the hot working of metals contain graphite by virtue of its good lubricating properties. It will be noted however that graphite suffers from considerable disadvantages such as for example the absorption of graphite carbon into the worked metal surface, whereby the composition and the properties of the metal surface can be altered. In addition graphite is undesirable for reasons relating to working hygiene as the graphite powder is easily atomised into the ambient atmosphere and represents a health risk by breathing in the graphite powder for people working in the vicinity.
Furthermore by virtue of their physical properties and grain sizes many known lubricants do not have good trickle and flow characteristics. A coarse material involving large grain sizes frequently results in inadequate and irregular covering of the metal surface and thus poor scale reduction. Known fine-grain materials of small grain sizes, for example less than 50 μm frequently have a tendency to form lumps, in particular upon storage, and for that reason it is only with difficulty that they can be sprayed in powder form onto the metal surface. A finer grain size however would afford the advantage that better layer formation can be achieved, but that advantage in known compositions of fine grain size is nullified again by the severe tendency to form lumps.